1. Field of the Invention
The invention relates to communication systems for serving mobile subscribers and, more particularly, to the routing of calls to mobile subscribers within a communications network.
2. History of the Related Art
In certain radio communication systems, such as cellular radio networks, each mobile station is identified by a unique mobile identification number (MIN) and is associated with a particular geographic area served by a particular home exchange. When the mobile station moves out of the area served by its home exchange and "roams" into other exchanges connected within the same radio network, a plurality of databases maintain information as to the current location and operation state of the mobile. For example, associated with each exchange, i.e., mobile services switching center (MSC), there is a home location register (HLR) which stores data defining the features to which each subscriber who has that MTSO as its home exchange has subscribed along with data identifying the current location within the network of each such subscriber. Similarly, each exchange also maintains a database listing each visiting or "roaming" mobile station currently operating within its exchange area along with indicia including the current busy state of that subscriber and its current cell or location area position.
Conventionally, a call intended for completion to a mobile subscriber arrives from the PSTN at an exchange connected as part of the cellular network [the gateway exchange (G-MSC)]. The gateway exchange sends a routing request message to the home location register of the mobile station for which the call is intended. From its database the HLR identifies the particular exchange within which the mobile station is currently located and forwards the routing request on to that exchange (the visited exchange). The visited exchange then returns to the gateway exchange a routing number identifying the mobile subscriber to which the call is directed within the visited exchange. The gateway exchange then seizes a voice circuit and routes the call from the gateway to the visited exchange. The mobile is paged within the visited exchange and the call completed.
A substantial improvement upon this prior art technique of completing calls to roaming mobile subscribers within a cellular communication network is shown in U.S. Pat. No. 5,282,242, hereby incorporated by reference, in which the function of call set-up to the mobile station is performed before the call is routed via voice circuits to the visited exchange. In this technique, when a visited exchange receives a routing request message from a gateway exchange indicating its desire to complete a call to a mobile station roaming within the visited exchange, the visited exchange first pages the mobile station and places it on a voice channel prior to returning a routing number to the gateway exchange. In this way, the expensive voice communication circuits between the gateway and visited exchanges are not tied up during the period when the visited exchange is attempting to locate the mobile station within its cellular coverage area. This technique also eliminates the possibility of wasted voice circuit time in the event the mobile station cannot be located or cannot be placed upon a voice channel because of difficulties with radio transmission at that particular time. This technique has resulted in substantial advantages and cost savings to mobile system operators.
An additional concept which has assumed increasing importance in the operation of cellular radio systems is that of the existence of local access and transport areas (LATAs). When the Bell system was broken up during the '80s with long distance service being allocated to AT&T and local service being allocated to the regional Bell operating companies (RBOCs), the United States was divided into a number of LATAs. The Final Judgment Rules under which local and long distance services are allocated between AT&T (as well as MCI and Sprint) as long distance carriers and the RBOCs as local service providers include the basic principle that the RBOCs are only allowed to provide service which originates and terminates within the same LATA. If a call originates in one LATA and is terminated in another LATA, even if the LATAs are adjacent to one another, the service must be completed through one of the three long distance carriers.
More recently, the local/long distance service allocation rules have been further modified to provide that automatic roaming of cellular subscribers within the national cellular network must be performed in accordance with the same rules separating local and long distance carriers. Thus, automatic roaming must ensure that when a call is routed from a gateway exchange to a roaming subscriber in a visited exchange which is busy, an interexchange carrier is used to return a busy tone from the visited exchange to the calling subscriber. In addition, interexchange carrier voice circuits must be used to determine whether or not a roaming mobile is located in a particular visited exchange (rather than using private data circuits as contemplated in U.S. Pat. No. 5,282,240) if the gateway exchange and the visited exchange are located in different LATAs. One way of dealing with this issue is to treat all visited exchanges and gateway exchanges as if they are in different LATAs and consequently disable all busy return procedures and location before routing procedures in the network. However, this procedure also prevents the proper and appropriate usage of both of these money-saving techniques in the case of calls which both originate and are completed in the same LATA even though the call is between two different exchanges, one serving as the gateway and one serving as the visited exchange within the same LATA.
The system of the present invention enables the efficient and cost effective implementation of busy indication, call set up before routing, and call forwarding on busy which is consistent with the current regulatory requirements of automatic roaming call set up within a multi-LATA mobile cellular network.